This invention relates to telecommunications, and more specifically to a spectrum management method for use with cable modems.
Within recent years, Cable TV companies have expanded their product offerings beyond traditional cable TV video services to include such offerings as internet data, voice, both switched circuit and IP, and active gaming services. Further, in an effort to remove the heavy cost of equipment from their balance sheets, these companies are attempting to move the market toward a retail purchase model of most in-home equipment.
As a result of this effort, cable companies allied themselves together to drive the development of the Data-Over-Cable Service Interface Specifications, DOCSIS, standards. The goal of this effort was to allow many vendors to produce equipment that would interoperate regardless of the manufacturer. Achieving this goal would greatly assist the cable companies in moving the market toward a retail model where end users purchase their own equipment. This means that modems would work in any cable infrastructure. In addition, it addressed a historical problem for cable companies in that they were no longer tied to a single source manufacturer for infrastructure equipment.
In order to provide data services to end customers, data must be able to travel in both directions: from the xe2x80x9cinternetxe2x80x9d to the customer, commonly called the forward-path or downstream, and from the customer to the xe2x80x9cinternetxe2x80x9d, commonly called the return-path or upstream. A substantial portion of the DOCSIS specification is dedicated to defining the protocols which are used to support providing data and Voice over IP services in both directions. However, one serious omission is present within the DOCSIS specification.
The forward-path direction of the Cable TV Plant functions as a fairly clean environment. This is entirely due to the fact that the traditional television set receiver utilizing an analog video demodulator requires a very high Carrier-to-Noise ratio, C/N, to produce a quality picture that satisfies the customer. Thus the net result regarding the forward path direction is a Carrier-to-Noise ratio which is sufficiently high enough to support both of the downstream DOCSIS digital modulation schemes, i.e. 64QAM and 256QAM.
The return-path on the other hand is a very hostile environment. The return path direction is not monitored by each household monitor for signal quality as is the case in the Forward Path direction. In essence, the only receivers available in the return path are the few Cable Modem Termination System, CMTS, digital receivers located at the CATV Head-end.
The hostility of the return path is a result of many factors. Since the CATV network today utilizes a tree and branch topology, there are numerous return path branches that are combined prior to the CMTS digital demodulator receiving the signal. Since it is a tree and branch topology design, all signals, whether they are Ingress Noise, Impulsive Noise, Continuous Wave, CW, type noise such as generated by Shortwave radio, or burst coherent signals such as generated by amateur radio and Citizens Band radio, all are combined prior to arriving at the CMTS digital receiver.
Because of the multiple path combining at the CMTS receiver and lack of customer complaints to the Cable TV operators to improve the signal path, the return path is extremely hostile to any type of communications transmission. More importantly, wideband digitally modulated signals are constrained to operate under the DOCSIS communications protocol. This means that burst transmission TDMA multiplexing is particularly vulnerable. Given the fact that there has not been a multitude of customers to serve as test sets, and given the fact that coaxial cable in any Cable TV network is optimized for the forward path direction, it should not come as too much of a surprise that the return path signal quality metrics are effectively still in their infancy.
Not only does the return path suffer from the summation or combining of return path branches, it also suffers from the fact that the return path has largely never been evaluated for known steady impairments such as micro-reflections, group delay, and system non-linearity. While some of these impairments such as micro-reflections and group delay could reasonably be controlled with proper return path alignment techniques such as a wideband sweep alignment technique, the impact due to non-linearity is generally speaking not well understood, and therefore continues to be largely ignored.
While the DOCSIS specification provides for a great deal of flexibility relative to the type of modulation scheme, symbol rates, degree of forward error correction, and frequency selection which may be used in the return-path, this protocol has failed to put the necessary provisions within the specification to allow for dynamically monitoring the return path-spectral quality in a timely fashion and adapting the return-path so as to maximize the data service provided within these hostile environments.
Thus, the hostile nature of the return-path makes spectrum management an absolute necessity, especially when considering such QOS-sensitive data services as Voice over IP. When constrained by the DOCSIS protocol, significant delays and data throughput impacts will be experienced as a result of probing the return-path spectrum. To date, no system has been developed which reduces these delays to adequate levels.
In order to monitor the spectral quality of the return path, signal-to-noise-ratio measurements must be performed which reflect both active signal noise characteristics as well as signal distortion impacts. Assessing signal distortion impact requires that a transmit burst of adequate duration and spectral quality be provided from various points within the cable plant infrastructure.
The DOCSIS specification does not support the functionality to allow modems to be forced to transmit the required signal on a frequency of interest in a timely manner. Further, the DOCSIS specification allows for a large channel retune time, in excess of 100 milliseconds, before being required to transmit on the return path. When taken together, these two factors result in a drastic impact on active data services when attempting to assess alternate spectral areas.
If an effort is made to monitor the spectral quality continually, then maximum achievable bit rates will be impacted. Similarly, if these assessments are only performed when the active communication channel degrades, then a large dropout of service will result as the system attempts to evaluate a significant area of spectrum before adapting the return-path configuration. As quality-of-service sensitive data services such as Voice-over-IP increase in deployment, the ability of the system to automatically respond to dynamic-hostile environments in a timely fashion will become even more critical.
One solution to this problem is when an active channel degrades, randomly hop to another section of the spectrum in hopes that it can support the data service, a trial-and-error approach which has serious consequences when the new spectrum is not capable of supporting the data traffic.
In order to be able to reliably hop to a so-called xe2x80x9cclean channelxe2x80x9d when an actual channel becomes degraded, in the subject method unused portions of the return path spectrum are continuously monitored in the background to be able to ascertain channel quality. This is accomplished through the use of a single parallel receiver which polls the cable modems at various taps. When an active channel becomes degraded, a reference table of unused upstream channels and their signal characteristics is consulted. When a suitable channel is found, the active channel is switched to this one. It will be noted that selecting the unused upstream spectrum for monitoring permits simultaneous data transmission on the active channels.
In order to monitor the unused portion of the spectrum, a polling signal is transmitted to a modem selected to transmit a reference signal on an unused upstream channel. The reference signal is such as to support both active noise and signal distortion measurements in the upstream direction. In order to transmit the reference signal, the selected modem is retuned to the unused channel. Thereafter the parallel receiver is used to measure various parameters of the unused channel. These parameters may include: slicer error, amplitude distortion, phase distortion, and power spectral noise characteristics as described in U.S. patent application Ser. No. 09/470,890 filed Dec. 22, 1999, entitled METHOD AND APPARATUS FOR AUTOMATED CORRELATION OF DIGITAL MODULATION IMPAIRMENT assigned to the assignee hereof and incorporated herein by reference; and Docket Number PD05944AM, U.S. patent application Ser. No. not available, filed May 15, 2000 entitled SLICED BANDWIDTH DISTORTION PREDICTION assigned to the assignee hereof and incorporated herein by reference.
The resulting measurement permits reassigning the active channel to this unused channel assuming the unused channel is better than the degraded one. The measurement is based on unused spectrum. In one embodiment, the modem selected depends on modem service activity and tap location. The selected modem will be from the worst performing tap. Historically, the worst performing taps in each coaxial run tend to be the lower loss taps. How the worst performing taps are ascertained will be described hereinafter in a section entitled Tap Performance.
If a good upstream channel can be found for the modem at this worst tap, then in all probability it will be good for modems at other better performing taps.
The subject invention thus provides for an approach which allows the return-path spectrum to be monitored xe2x80x9cin the backgroundxe2x80x9d. As will be seen, the subject approach is compatible with the DOCSIS protocol. In one embodiment, monitoring is accomplished by the use of a single parallel receiver while not degrading the services provided by the cable operator. Thus, there is no impact on active data services. In addition, since the spectral monitoring is constantly being performed, the subject system provides a mechanism by which a backup xe2x80x9ccleanxe2x80x9d channel is found and used immediately upon active channel degradation. Further, the subject invention does not require any additional devices beyond DOCSIS cable modems to be installed within the cable plant.
Note that U.S. patent application Ser. No. 5,608,727 issued Mar. 4, 1997 for xe2x80x9cMethod and System for Management of Frequency Spectrum Among Multiple Applications on a Shared Mediumxe2x80x9d assigned to the assignee hereto and incorporated herein by reference describes a system for frequent spectrum management; and that U.S. patent application Ser. No. 09/052,224 filed Mar. 31, 1998, entitled xe2x80x9cSystem, Device, and Method for Selecting A Channel in A Multichannel Communication Networkxe2x80x9d describes a system for channel selection based upon a reference signal.
In summary, in a cable modem system a method is provided for continuously monitoring the upstream channel to assess what unused upstream channels will better support data communication in the event that an active upstream channel degrades, with monitoring of the upstream path by using passive or unused channels not impacting active data transfer. In one embodiment, a single spectrum assessment receiver polls cable modems at various taps and has the selected modem retune itself to an unused channel and transmit a reference signal back to the spectrum assessment receiver. An analysis is made and an impaired active channel is identified to ascertain if it has degraded to an unacceptable level. If so, it is switched or hopped to this unused channel if by switching an improvement can be realized. Polling is accomplished by selecting the modem at the worst performing tap and ascertaining after one or more retunes which unused channel is best. The quality of all of the taps is maintained in a continuously-updated table to permit proper tap/modem selection.